The invention relates to a fuel metering system for internal combustion engines and includes a fuel container and a fuel line leading from the fuel container to the induction tube of the engine. Through this fuel line, fuel is added to the air flowing through the induction tube in proportion to the air flow rate. The amount of fuel is controlled by the pressures which prevail in the fuel container and in the induction tube. The pressure prevailing in the fuel container may be changed by means which operate in dependence on engine parameters, and especially in dependence on the output signal of a measuring sensor which determines the composition of the exhaust gas. The air space within the fuel container is connected by air conduits with induction tube regions lying upstream and downstream of the throttle valve and the flow cross section of these air conduits is variable in dependence on the output signal of the measuring sensor.
In view of the very high technical demands being made nowadays on such fuel metering systems, they must automatically create a favorable fuel-air mixture in an internal combustion engine under all operational conditions. In this way, the fuel can be combusted as completely as possible so that, while maintaining the highest possible power, or the lowest possible fuel comsumption, as the case may be, the engine produces no or relatively few toxic exhaust components. For this purpose, the fuel quantity added to the combustion air must be precisely regulated and adapted to the requirements of each and every operational state of the internal combustion engine. Thus the average most favorable fuel-air ratio must be made changeable in dependence on engine parameters and, especially on the exhaust gas characteristics and this is done, in the fuel metering system described above, by changing the pressure in the air space of the fuel container. In known fuel metering systems of this type, when the engine operates under low load or low rpm, control fluctuations occur and the amplitudes of these control fluctuations exceed the values which the catalyzer in the exhaust system is capable of correcting. One reason for these control fluctuations is that droplets of fuel are formed on the surface of the throttle valve and on the wall of the induction tube lying upstream of the throttle valve and these droplets enter the passing fuel-air mixture only after considerable delay even when the air flow velocity is changing.